\begin{tikzpicture}
  [node/.style={circle, draw}]
  \matrix 
  [matrix of nodes,
  nodes in empty cells,
  row sep={4em,between origins},
  column sep={3em,between origins}]
  {
    &&\node[node](n7){7};&&\\
    &&\node[node](n5){5};&&\node[node](n6){6};\\
    &\node[node](n4){4};&&\node(n3)[node]{3};&\\
    &&\node[node](n1){1};&&\node[node](n2){2};\\
  };
  \node[right=of n7,xshift=-2.5em] (n7r) {\bcancel{$t_4$ | $b$}};
  \node[above=of n7r,yshift=-3em] {$b$ | $t_4$};
  \node[left=of n7,xshift=2.5em] {+};
  \node[right=of n5,xshift=-2.5em] (n5r) {\bcancel{$t_3$ | $y$}};
  \node[above=of n5r,yshift=-3em] {$y$ | $t_3$};
  \node[left=of n5,xshift=2.5em] {*};
  \node[right=of n6,xshift=-2.5em] {$b$};
  \node[right=of n4,xshift=-2.5em] {$a$};
  \node[right=of n3,xshift=-2.5em] (n3r) {\bcancel{$t_2$ | $x$}};
  \node[above=of n3r,yshift=-3em] {$x$ | $t_2$};
  \node[left=of n3,xshift=2.5em] {-};
  \node[right=of n1,xshift=-2.5em] {\bcancel{1 | $y$}};
  \node[right=of n2,xshift=-2.5em] {15 | $t_1$};
  \draw[-] (n7) [out=-20,in=130] to (n6);
  \draw[-] (n7) [out=-150,in=90] to (n4);
  \draw[-] (n5) [out=-150,in=75] to (n4);
  \draw[-] (n5) [out=-30,in=105] to (n3);
  \draw[-] (n3) [out=-150,in=75] to (n1);
  \draw[-] (n3) [out=-30,in=105] to (n2);
\end{tikzpicture}